Future wireless sensor networks will comprise sensor nodes which occupy a volume of typically a few cm3. The scaling down of batteries for powering these sensor nodes faces technological restrictions as well as a loss in storage density. Energy scavengers based on the recuperation of wasted ambient energy are a possible alternative to batteries. Several scavenger concepts have been proposed, based on the conversion of thermal energy, pressure energy or kinetic energy.
Kinetic energy scavengers convert energy in the form of mechanical movement (e.g. in the form of vibrations or random displacements) into electrical energy. For conversion of kinetic energy into electrical energy, different conversion mechanisms may be employed, for example based on piezoelectric, electrostatic or electromagnetic mechanisms. Piezoelectric scavengers employ active materials that generate a charge when mechanically stressed. Electrostatic scavengers utilize the relative movement between electrically isolated charged capacitor plates to generate energy. Electromagnetic scavengers are based on Faraday's law of electromagnetic induction and generate electrical energy from the relative motion between a magnetic flux gradient and a conductor.
Electrostatic energy conversion is based on a variable capacitance structure that is driven by mechanical vibrations and oscillates between a maximum capacitance and a minimum capacitance, thereby converting kinetic energy into electrical energy. In micromachined electrostatic scavengers the relative movement between electrically isolated capacitor plates is obtained by providing a movable electrode and a fixed electrode. The electrodes may have a comb structure comprising an array of fingers, and both electrodes may be interdigitated.
Relative movement between the capacitor plates may comprise changing the overlap area of the fingers (in-plane overlap scavenger) or changing the gap between the fingers (in-plane gap-closing scavenger). The energy output per cycle increases with increasing maximum capacitance (and thus e.g. with increasing size of the electrodes) and is proportional to the mass of the oscillating structure. Therefore, maximizing the mass of the system is an important design consideration. For example, a suspended mass acting as a seismic mass may be connected to the movable electrode.
In prior art in-plane gap-closing devices, e.g. as described by G. Despesse et al. in “Fabrication and characterization of high damping electrostatic micro devices for vibration energy scavenging”, Design, Test, Integration and Packaging of MEMS and MOEMS, 2005, the fingers of the movable electrode are horizontally connected to the suspended mass, meaning that they lay substantially in the same plane as the mass. Both the interdigitated electrodes and the suspended mass are for example made from the same functional silicon layer of an SOI substrate. As the seismic mass occupies a given space, e.g. determined by the designated resonance frequency, the electrodes can only be enlarged to cover the residual area of the die. Therefore, for such a device, there is a need for making a trade-off between the size of the seismic mass and the size of the electrodes. Additionally the length of the electrode fingers can not exceed a certain value, as set by structural stability requirements.
In “MEMS design and fabrication of an electrostatic vibration-to-electricity energy converter”, Microsystem Technologies, Vol.13, pp 1663-1669, 2007, Y. Chiu et al. present an in-plane gap-closing electrostatic scavenger wherein the seismic mass is increased by providing an externally attached mass (a steel ball). The addition of a separate mass increases the effective mass considerably. This leads to the desired operation at low frequencies (<100 Hz). However, also in this design a trade-off is made between the space needed for attaching the externally attached mass and the size of the electrodes. Moreover, the center of the external mass is located at a relatively large distance above the plane of the actual silicon structure. Therefore, in-plane vibrations may lead to a torque moment acting on the supporting silicon microstructure, which may lead to undesired contact between the movable and the fixed silicon electrodes. Furthermore, by adding the separate mass the compactness and thus the power density of the device are significantly reduced.
A vibration energy scavenger has its maximum power output when input vibrations closely match its resonance frequency, which is influenced by the material properties and the dimensions of the scavenger's parts. The fabrication of micromachined energy scavengers may lead to differences in their mechanical characteristics and thus their resonance frequency. Furthermore the dominant ambient vibration frequency may shift over time. The vibration energy scavenger structure may stay out of resonance when the input vibration frequency changes, resulting in very low power generation or no power generation at all. Therefore, it would be advantageous to have vibration energy scavenger with a tunable resonance frequency.
In “Resonance tuning of piezoelectric vibration energy scavenging generators using compressive axial preload”, Smart Materials and Structures 15, 2006, 1413-1420, Leland et al describe a method wherein axial forces are applied on the suspension of a macroscopic double-clamped beam for changing the effective stiffness of an oscillating structure. In this approach, a simply supported piezoelectric bimorph is used as an active element, with a proof mass mounted at the bimorph's center. A variable compressive axial preload is applied to the bimorph, reducing its stiffness and thus the resonance frequency of the device. This approach uses an externally applied force. Furthermore, it may be difficult to fabricate the system proposed by means of micromachining methods.
In V. R. Challa et al, “A vibration energy harvesting device with bidirectional resonance frequency tenability”, Smart Materials and Structures 17, p. 015035, 2008 a magnetic force is used to alter the overall stiffness and thus the resonance frequency of an energy harvesting device. Resonance frequency tuning is based on adjusting the position of permanent magnets. The position of the magnets is controlled externally. Resonance frequency tuning of a MEMS resonator using this approach may be unfeasible.
Resonance frequency tuning of oscillating structures is used in various MEMS resonators. More in particular, for resonators based on a comb structure (comprising interdigitated fingers) methods for post-fabrication resonance frequency tuning have been proposed. For example, in “A closed-form approach for frequency tunable comb resonators with curved finger contour”, Sensors and Actuators A 141, p 523-529, 2008, K. B. Lee et al describe frequency tunable resonators with curved finger contours. However, fabricating the ideal design is not easy and it may require a lot of space. In “Vertically-shaped tunable MEMS resonators”, Journal of Microelectromechanical Systems, Vol 17, No 1, p. 85, 2008, B. Morgan et al describe a tunable MEMS resonator with vertically-shaped comb fingers. However, for fabricating such structures gray scale lithography is needed. In “A frequency selective silicon vibration sensor with direct electrostatic stiffness modulation”, Analog Integrated Circuits and Signal Processing, 37, pp 35-43, 2003, D. Scheibner et al describe MEMS comb resonator wherein electrostatic resonance tuning is implemented by a comb system with a linearly varying finger length.